This invention relates to brazing alloy sheet of the kind comprising an aluminium-based core and on at least one side a cladding of an aluminium-based brazing alloy containing silicon as the main alloying ingredient. The brazing alloy sheet is characterised by having excellent corrosion resistance, and also improved sag resistance and post-brazed strength.
U.S. Pat. Nos. 5,037,707 and 5,041,343 (both assigned to Alcan) describe brazing alloy sheet of this kind, in which the core is typically a 3000 series alloy (from the Aluminum Association Register) containing about 0.7% to about 1.5% Mn and about 0.1% to about 0.6% Cu. The Fe concentration is no more than 0.4% and the Si concentration no more than 0.15%, and it is control of these two components that is primarily responsible for the good post-braze corrosion resistance properties. This long-life core alloy is clad on one or both sides with an Al-Si brazing alloy. The clad alloy is hot rolled, then cold rolled, preferably without any interannealing and preferably with minimum final annealing of the rolled sheet.
Both homogenisation and interannealing are detrimental to the corrosion resistance of the brazed product. The preferred copper content of the core alloy is from about 0.2% to about 0.4%.
In the resulting brazing alloy sheet, the Mn is preferably present substantially in solid solution and/or in the form of a fine precipitate of Mn-containing particles in the range of 0.03-0.1 .mu.m. During brazing, there is formed a band, known as the brown band, of densely precipitated particles containing Al, Mn and Si. This band is typically a few tens of microns thick, at the surface of the core adjacent the cladding layer, with the silicon in the band having migrated from the cladding layer, and the band having substantially less Mn in solid solution than in the core alloy outside the band. The band is responsible for the excellent corrosion resistance of the clad side of the brazed product. Close control of the Fe and Si contents of the core alloy are required in order to obtain a satisfactory brown band.
The heat exchanger market requires aluminium brazing sheet that offers a balance of properties: corrosion resistance, brazability, strength especially post-braze strength, and formability. Often the property balance is a compromise between competing processes or microstructures. The alloys described in the aforesaid U.S. patents have excellent external corrosion resistance with adequate strength and sag resistance.
This invention is based on the discovery that alloys of this kind having higher levels of Cu, in the range of 0.5-1.0%, have surprisingly improved properties. Significant properties discussed below include strength, sag resistance, corrosion resistance and processability.
U.S. Pat. No. 4,761,267 (Sky Aluminium) describes brazing sheet comprising a core alloy consisting of 0.5-1.0% Cu, 0.6-1.0% Mn, 0.10-0.30% Ti, 0.3% or less Fe, &lt;0.10% Si, balance Al. To one or both sides of this core alloy are applied claddings of Al--Si--Mg filler metal. A sacrificial anode metal may be applied to one side. The function of Cu is stated to shift the electrode potential of the core alloy, thereby allowing the cladding to act as a sacrificial anode; and Cu is used despite the fact that it degrades the corrosion resistance of the core material per se. Ti and Mn are both indispensable for attaining satisfactorily large cathode polarisation. Fabrication techniques are not critical: preferably a homogenising treatment at a high temperature and for a long period of time is not carried out before the hot rolling. The presence of a brown band (in the surface region of the core alloy after brazing) is not mentioned; a comparison of alloys 14 and 15 (Tables 1, 2 and 3) suggests that no brown band was present; otherwise the corrosion resistance of alloy 14 would have been 4 to 6 times better than alloy 15 (AA3003).
SAE Technical Paper 930149 teaches the importance of Ti in reducing corrosion of brazing sheet core alloy and in confining corrosion to surface layers.
Japanese Kokai 61-82992 (Furukawa Aluminium) describes aluminium alloy brazing sheet with a core containing 0.25-1.0% Cu. 0.3-1.5% Mn, &lt;0.2% Si and &lt;0.2% Fe. There is again no reference to any brown band being formed after brazing. The manufacturing sequence includes a step of homogenising prior to hot rolling, and the corrosion results reported are so poor as to suggest that no brown band was present in the brazed product.
Japanese Kokal 195240/1988 (Furukawa Aluminium) describes an Al brazing sheet comprising 0.3 to 0.9% Cu; 0.5 to 1.5% Mn; less than 0.2% Si; and 0.2 to 1.0% Fe; balance Al, the crystalline particle size of the material being 50 to 150 .mu.m.
GB 2,159,175A (Sumitomo Light Metal) describes fin stock material of an alloy containing 0.6 to 1.5% Mn; 0.1 to 1.0% Cu; 0.1 to 0.75% Mg; less than 0.30% Si less than 0.8% Fe; balance Al.